1. Field of the Invention
The present invention relates to a magnetic disk medium, a reticle for reduction exposure used when manufacturing the magnetic disk medium, and a magnetic recording and reproducing apparatus.
2. Related Art
In the magnetic recording apparatus, magnetic disk apparatus and hard disk drive apparatus, there is a problem that magnetic information recorded by a recording and reproducing head which moves relatively on a magnetic disk medium is apt to exert bad influence upon recording on adjacent tracks because of heat fluctuation, as the recording density on the magnetic disk medium increases with an increase in capacity. Against this problem, it is possible to use means for eliminating the above-described bad influence by physically separating magnetic materials in adjacent tracks included in the magnetic medium. Magnetic disk media thus including magnetic materials each having a shape of a recording track or a recording bit, or magnetic disk media having patterns in which a magnetic material is divided by non-magnetic materials are called patterned media. Since tracks are divided, the media are called discrete tracks as well sometimes. (For example, see U.S. Pat. No. 6,563,673 and JP-A 2004-110896(KOKAI).)
As for a technique for working on magnetic materials of such patterned media, a magnetic disk medium can be generated by drawing desired magnetic material patterns on an original disk by means of, for example, an original disk drawing system, generating an imprint stamper, and utilizing a nano-imprint method using the imprint stamper (see, for example, JP-A 2003-157520(KOKAI)). In fabrication of the original disk, its patterns can be formed by exposing a photosensitive resin to chemical rays such as a mercury lamp, ultraviolet rays, an electron beam or X-rays. In particular, a technique of irradiating directly an electron beam is desirable to form magnetic disk patterns which need fine pattern drawing in order to achieve a high recording density.
For example, if an original disk is fabricated by conducting EB (Electron Beam) irradiating on a positive type resist film, exposed parts become concave parts after development. If this original disk is electroformed to form a stamper, the exposed parts become convex parts. Therefore, an exposed discrete track groove becomes a convex arc when a stamper is formed. In addition, convex parts on the stamper surface are transformed to concave parts on a resist film by a nano-imprint process. In subsequent etching processing, a magnetic film or a substrate on a medium provided under the concave parts is etched. As a result, patterns of magnetic materials and non-magnetic materials or uneven patterns of magnetic materials are formed on the surface of the magnetic disk medium.
When conducting recording or reproducing on the magnetic disk medium by using a magnetic recording and reproducing head, recording or reproducing is conducted on a desired position on the medium surface by relatively moving the magnetic recording and reproducing head on the surface of the magnetic disk medium.
In order to move the recording and reproducing head to a desired position on the magnetic disk medium, a positioning servo area is present on the magnetic disk medium. At the time of recording or reproducing, a position of the recording and reproducing head on the magnetic disk medium is grasped on the basis of a position signal obtained from a reproducing head when the recording and reproducing head crosses the servo area. The recording and reproducing apparatus controls the position on the basis of the obtained position information, and moves the recording head as far as a desired recording position.
In the patterned media, it is desirable to generate a signal in the servo area collectively together with a data area as magnetic material patterns from the viewpoint of holding down the cost when fabricating the medium and obtaining a high position precision by generating the servo positioning signal and the recording patterns collectively.
The magnetic pattern shape in the servo area used in ordinary HDDs (hard disk drives) will now be described. The servo area includes at least a preamble area, an address area and a positioning burst area (or a burst area).
The preamble area functions to adjust an amplification factor of a signal amplifier and make the amplitude constant before reading out servo data. The preamble area precedes the address area and the positioning burst area in the servo area along the head sweeping direction. The preamble area is formed of a plurality of linear patterns which cross the track direction, so as to obtain a similar signal no matter which track position the head lies in.
The address area has information of a track number and a sector number, and the address area indicates a track position in which the head lies. Address information is written in the Gray code so as to make it possible to read data even if the head shifts to an adjacent track during seeking.
The burst area has a structure in which two or more pattern areas are arranged in the track direction. In each of the pattern areas, magnetic patterns are arranged at equal intervals in a direction crossing the tracks. A signal supplied from the burst area has deviation information in the track of the head position.
When conducting recording or reproducing on the patterned medium by using a floating head, an air flow between the head and the medium exerts great influence on the floating stability of the head. When generating a preamble signal by using a patterned medium, a recording area has uneven groove patterns which are parallel to the tracks whereas the preamble patterns are uneven groove patterns which cross the tracks. When the head moves from the recording area to the servo area at the time of recording and reproducing, the medium shape under the head passes from the uneven groove patterns parallel to the track direction to the uneven groove patterns crossing the tracks, the air flow right under the head varies, and the floating stability of the head is lost. Grooves between tracks generate a streamline flow in the circumferential direction, different external forces are given to the left and right of the head, and the floating stability of the head is hampered. Because of them, there is a problem that the head cannot read the servo signal or the data signal properly.
On the other hand, when the original disk is exposed to an electron beam, there is a precision error of the drawing position in the electron beam lithography system. When drawing fine patterns required for high recording density, pattern collapse caused by an error in the drawing precision occurs. As one of techniques for solving this problem, reduction projection exposure using a reticle can be used.
Even if there is position fluctuation to some degree in the patterns on the reticle, the magnitude of the fluctuation is also reduced by the reduction projection. Use of the electron beam reduction exposure brings about such an advantage.
As main electron beam reduction projection exposure techniques, the SCALPEL (Scattering with Angular Limitation in Projection Electron Lithography) technique described in S.D. Berger et al., Applied Physics Letters, 57, 153(1990) and the PREVAIL (Projection Exposure with Variable Axis Immersion Lens) technique described in Japanese Patent No. 2829942 can be mentioned. As masks used in these techniques, there are the stencil type and membrane type. When the stencil mask is used, the electron beam is passed through opening parts, and scattered at non-opening parts. The membrane mask includes a membrane formed of a light element through which the electron beam is transmitted easily, and a heavy metal pattern layer formed on the membrane to scatter the electron beam. As the membrane, silicon, a silicon nitride film or the like is used. As the pattern part, chromium (Cr), tungsten (W) or the like is used.
Since the opening part through which the beam of the stencil mask is transmitted pierces, low scattering or chromatic aberration does not occur unlike the membrane mask. Since an omission occurs in the toroidal patterns because of its structure, however, masking cannot be conducted. Therefore, a complementary mask is used, and a technique of coping with the problem by conducting exposure a plurality of times is used. Such coping poses a problem in position alignment and throughput. Furthermore, cantilever patterns and long and narrow patterns are apt to become low in mechanical strength and be broken. On the other hand, the membrane mask can be formed in the toroidal patterns as well, because there is the membrane part. However, the toroidal patterns and the cantilever patterns tend to become weak in mechanical strength in the same way. Furthermore, in the membrane mask, low scattering is caused even it is slight when the electron beam is transmitted through the membrane, resulting in a drawback of occurrence of chromatic aberration.
When attempting collective projection exposure on preamble patterns in patterned media, it is necessary to prepare a mask having openings of a plurality of parallel continuous strip-shaped pattern portions. However, such slender openings have a problem that mask flexion causes optical hindrance in projection and the mask strength becomes weak.
In the patterned media, the floating stability of the magnetic recording and reproducing head poses a problem as described above.
In addition, in the reduction projection exposure process used when fabricating a patterned medium, there is a problem in strength of long and narrow patterns such as the preamble part of the reticle for exposure.